Forgive me for being ignorant, but what, exactly, is a scrim? The best I can decipher is that it's a background...but in that case why call it a scrim?

I'm familiar with the term scrim from theater. It is a thin, see-through curtain that can allow the action to take on special effects. For example, the main action can be in front of the scrim with a dream sequence behind the scrim.